This disclosure relates generally to the field of wellbore drilling through subsurface formations. More specifically, the disclosure relates to methods for reducing undesirable modes of motion that induce undesirable vibration levels in a drill pipe “string” used to drill such wellbores.
Drilling wellbores through subsurface includes “rotary” drilling, in which a drilling rig or similar lifting device suspends a drill string which turns a drill bit located at one end of the drill string. Equipment on the rig and/or an hydraulically operated motor disposed in the drill string rotate the bit. The drilling rig includes lifting equipment which suspends the drill string so as to place a selected axial force (weight on bit—“WOB”) on the drill bit as the bit is rotated. The combined axial force and bit rotation causes the bit to gouge, scrape and/or crush the rocks, thereby drilling a wellbore through the rocks. Typically a drilling rig includes liquid pumps for forcing a fluid called “drilling mud” through the interior of the drill string. The drilling mud is ultimately discharged through nozzles or water courses in the bit. The mud lifts drill cuttings from the wellbore and carries them to the earth's surface for disposition. Other types of drilling rigs may use compressed air as the fluid for lifting cuttings.
The forces acting on a typical drill string during drilling are very large. The amount of torque necessary to rotate the drill bit may range to several thousand foot pounds. The axial force may range into several tens of thousands of pounds. The length of the drill string, moreover, may be twenty thousand feet or more. Because the typical drill string is composed of threaded pipe segments having diameter on the order of only a few inches, the combination of length of the drill string and the magnitude of the axial and torsional forces acting on the drill string can cause certain movement modes of the drill string within the wellbore which can be destructive. For example, a well known form of destructive drill string movement is known as “stick-slip”, in which the drill string becomes rotationally stopped along its length by friction and is caused to “wind up” by continued rotation from the surface. The friction may be overcome and torsional release of the drill string below the stick point may cause such rapid unwinding of the drill string below the stick point so as to do damage to drill string components. Stick slip may be particularly damaging when certain types of directional drilling devices, called “rotary steerable directional drilling systems” are used. Stick-slip may cause undesirable vibrations that in turn could reduce the life of the drill string components such as bits, motors, MWD equipment, LWD equipment and the BHA.
There is a need for methods to reduce destructive modes of motion of a drill string during drilling. There is also a need for methods to reduce fatigue and wear of drill string and wellbore components during drilling.